U.S. Pat. No. 4,223,361 says that better wear resistance or durability is attained by combining two expedients of the prior art, namely, incorporating both granular (nonacicular) alpha-iron oxide particles and a liquid hydrocarbon lubricant in the magnetizable layers. The magnetizable layer of each of the examples comprises, based on 100 parts total weight, about 62 parts of acicular gamma-Fe.sub.2 O.sub.3 particles, 1.5 parts of granular alpha-Fe.sub.2 O.sub.3 particles, 6.5 parts carbon black, and 30 parts of binder. The gamma-Fe.sub.2 O.sub.3 used in the examples is said to have a particle size of about 0.3 micrometer and an axis ratio of about 8, while the granular alpha-Fe.sub.2 O.sub.3 has a particle size of about 1 micrometer. U.S. Pat. No. 4,223,361 says that the proportion of the magnetizable (gamma-Fe.sub.2 O.sub.3) particles is usually desired to be from 50:50 to 90:10 by weight (col. 4, lines 4-7) and that the amount of the granular alpha-Fe.sub.2 O.sub.3 particles is favorably from about 0.5 to 5% by weight of the magnetizable particles (col. 2, lines 28-30). The liquid lubricant is favorably used in an amount of about 0.5 part by weight or more to one part by weight of the alpha-Fe.sub.2 O.sub.3 particles (col. 2, lines 55-58). The liquid lubricant is a topical lubricant in that it is coated from solution onto the magnetic recording layers and apparently is absorbed into pores, since the patent warns that an excessive amount would exude to the surface to contaminate the magnetic head.
Among other patents wherein alpha-Fe.sub.2 O.sub.3 particles are included in a magnetizable layer is U.S. Pat. No. 4,425,400 (Yamaguchi et al.) which concerns video cassette recording tape and says nothing about diskettes. Even though the problems in the diskette art are very different from magnetic recording tape problems, it might be argued that U.S. Pat. No. 4,425,400 has teachings pertinent to the present invention because acicular alpha-Fe.sub.2 O.sub.3 particles are used in the magnetizable layer. That layer also includes an abrasive (head-cleaning) agent such as fused alumina which is illustrated in the drawings as granular or nonacicular. The invention of U.S. Pat. No. 4,425,400 differs from prior art only by substituting acicular alpha-Fe.sub.2 O.sub.3 for nonacicular alpha-Fe.sub.2 O.sub.3. Doing so is said to decrease head abrasion, decrease jitter, increase tape durability, decrease contact noise, and decrease contamination in use. Based on 100 parts by weight total, a typical magnetic recording layer of U.S. Pat. No. 4,425,400 comprises 65 parts acicular gamma-Fe.sub.2 O.sub.3 particles (long axis 0.9 micrometer, short axis 0.15 micrometer), 10 parts acicular alpha-Fe.sub.2 O.sub.3 particles (preferably of smaller thickness than the gamma particles), 3 parts abrasive agent particles, 3 parts carbon black, and 19 parts of binder. The videotapes of U.S. Pat. No. 4,425,400 include lubricants such as the oleic acid and silicone oil used in the examples. The lubricants are incorporated into the dispersion that is coated out to provide a magnetic recording layer and hence are dispersed lubricants, as opposed to the topical lubricant of U.S. Pat. No. 4,223,361. U.S. Pat. No. 4,425,400 teaches that these dispersed lubricants are generally used in a proportion of about 0.2 to 20 parts per 100 parts by weight of binder (col. 4, lines 38-40).
Japanese patent application No. Showa 56-34993, filed Mar. 11, 1981, by Hitachi Maxell K. K. concerns a magnetic recording tape which is said to have improved wear resistance. Diskettes are not mentioned. The application includes a tape, the magnetizable layer of which comprises acicular gamma-Fe.sub.2 O.sub.3 and alpha-Fe.sub.2 O.sub.3 particles, carbon black, and dispersed lubricant.
One reason why diskettes and magnetic recording tapes involve different problems is that tape recorders invariably erase before re-recording, whereas all diskette recorders overwrite data without an intermittent erase. In order to completely eliminate each previous signal, the coercivity (H.sub.c) of a diskette recording layer must be closely controlled. This involves close control of both the H.sub.c of individual magnetizable particles and of their proportion in the recording layer. For reliable overwrite in present-day standard diskette recorders, the magnetizable particles should provide a recording layer having an H.sub.c from 280 to 340 oersteds. In most diskettes now on the market, an H.sub.c within that range is provided by acicular gamma-Fe.sub.2 O.sub.3 particles comprising from 60 to 65 percent by weight of a magnetizable layer.
Another problem unique to a diskette is that it is rotated at high speed while a recording head is pressed against the recording disk. Since the head may remain in moving contact with a single track of the recording disk for prolonged periods, the magnetizable layer must have excellent wear resistance. This requires a lubricant content that may be greater than can be tolerated in a dispersion from which a magnetizable layer is formed, because such a large quantity of dispersed lubricant might injure the integrity of the binder. For this reason, diskettes usually employ a topical lubricant (as in U.S. Pat. No. 4,223,361), often in combination with minor amounts of dispersed lubricant, whereas many audio and video tapes employ only dispersed lubricants (as in U.S. Pat. No. 4,425,400).
Diskette specifications require the recording layer to be exceedingly smooth, i.e., to have a smoothness better than 0.1 micrometer peak-to-valley. Upon applying a dispersion of acicular gamma-Fe.sub.2 O.sub.3 particles in binder-forming material by techniques which will produce a usefully smooth coating, the resulting layer may have insufficient voids to absorb a desirably large amount of topical lubricant. As indicated in U.S. Pat. No. 4,223,361, any topical lubricant which is not absorbed by the magnetizable layer may contaminate the recording head.